The invention relates to projection lithography employing soft x-rays and in particularly to reticles that exhibit minimum thermal distortion during scanning. The invention is particularly suited for systems that use a camera that images with acuity along a narrow arc or ringfield. The camera employs the ringfield to scan the reflective reticle and translate a pattern onto the surface of a wafer.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the xe2x80x9cprojectingxe2x80x9d radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85 (xe2x80x9cAngstromxe2x80x9d) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning (e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015), available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term xe2x80x9cproximityxe2x80x9d), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength xcexxe2x89xa616 xc3x85 is required for 0.25 xcexcm or smaller patterning to limit diffraction at feature edges on the mask.
Use has been made of the synchrotron source in proximity printing. Consistent with traditional, highly demanding, scientific usage, proximity printing has been based on the usual small collection arc. Relatively small power resulting from the 10 mrad to 20 mrad arc of collection, together with the high-aspect ratio of the synchrotron emission light, has led to use of a scanning high-aspect ratio illumination field (rather than the use of a full-field imaging field).
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV (xe2x80x9cEUVLxe2x80x9d) lithography (a.k.a., soft x-ray projection lithography (xe2x80x9cSXPLxe2x80x9d)).
A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
Masks or reticles for EUV projection lithography typically comprise a silicon substrate coated with an x-ray reflective material and an optical pattern fabricated from an x-ray absorbing material that is formed on the reflective material. In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the x-ray absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle. Among the problems encountered in EUV projection lithography are point-to-point reflectivity variations. The art is in search of techniques to reduce reticle distortions.
The present invention is based in part on the recognition that thermal distortion in reticles can be significantly reduced by fabricating reticles exhibiting improved radiative cooling in vacuum systems. For example, this can be achieved by designing the nonactive regions of reflective reticles not to be coated with the high reflective material that is found on the surface of the active region where the pattern is formed. Alternatively, the nonactive regions can be coated with a high emissivity material. By employing emissivity engineering which involves the selective placement or omission of coatings on the reticle, the inventive reflective reticle fabricated will exhibit enhanced heat transfer thereby reducing the level of thermal distortion. Ultimately, the quality of the transcription of the reticle pattern onto the wafer is improved.
Accordingly, in one aspect, the invention is directed to a reflective reticle that includes:
substrate having an active region on a first surface of the substrate; and
at least one non-active region on a second surface of the substrate wherein each non-active region is characterized by having a surface that is formed of material that has an emissivity that is higher than that of the materials forming the active region surface.
In another aspect, the invention is directed to photolithography system that includes:
a source of extreme ultraviolet radiation;
means for collecting the radiation emitted from the source of extreme ultraviolet radiation and forming a light beam therefrom that is directed to an active region of a reflective reticle, wherein the reflective reticle includes:
(i) a substrate having an active region on a first surface of the substrate; and
(ii) at least one non-active region on a second surface of the substrate wherein each non-active region is characterized by having a surface that is formed of material that has an emissivity that is higher than that of the materials forming the active region surface; and
a wafer disposed downstream from the reflective reticle.
In a further embodiment, the invention is directed to a process for fabrication of a device comprising at least one element having a dimension xe2x89xa60.25 xcexcm, such process comprising construction of a plurality of successive levels, construction of each level comprising lithographic delineation, in accordance with which a subject active region of a reflective reticle is illuminated to produce a corresponding pattern image on the device being fabricated, ultimately to result in removal of or addition of material in the pattern image regions, in which illumination used in fabrication of at least one level is extreme ultra-violet radiation, characterized in that the process employs the inventive reflective reticle.
Modeling studies suggest that emissivity engineering can effectively reduce the distortions especially for reflective silicon reticles. For silicon reticles, simulations have shown an 82% reduction in total placement errors and a 25% reduction in residual placement errors.